The technology involved, initially described in U.S. Pat. No. 3,808,433, of Apr. 30, 1974, Fite and Myers, is directed to detection of small particulate matter and macromolecules by a process in which particles that impact onto a hot surface are partially or completely pyrolysed and a thermal and electrochemical equilibrium is established between the particles' atomic and molecular constituents and the surface. This equilibrium causes positive and/or negative ions to form at the surface which erupt as a burst of ions from that surface to be collected elsewhere as ion pulses which are counted and provide information regarding the impacting particles. Details regarding the types of materials suitable for hot surfaces and the sizes and chemical compositions of particles that may be thus detected are discussed in U.S. Pat. No. 3,808,433.
A technique whereby measurement of the direct current (dc) of ions leaving the hot surface provides a basis for deduction of additional information about the particulates is disclosed in U.S. Pat. No. 4,151,414 of Apr. 24, 1979, Fite and Myers, incorporated herein by reference. If, for instance, the dc reading is high, but the count rate is low, then it is likely that either the particles are very large or that they are relatively rich in surface-ionizable constituents.
From the disclosure of U.S. Pat. No. 4,162,404, it will be appreciated that it is not necessary to use pumps or fans to draw the particulate-laden air into a chamber for the purpose of impacting the particulates onto the hot surface if the air is already flowing at an appreciable speed such as in a duct, stack, pipe or the like. Thus, the detector can take the form of a particle counting and total dc ion current measurement.
The above mentioned patents are incorporated by reference herein as are related U.S. Pat. Nos. 3,973,121 and 4,093,855, for their disclosures relative to the background of the instant invention.
In one of the inventions described above, a platinum or platinum alloy wire heated resistively to a temperature suitable for surface ionization and which also caused emission of visible light was used as the sensor surface. To maintain the sensor surface at a constant temperature in a moving gaseous stream of varying speed and temperature, the visible radiation emitted by the platinum wire was electronically measured and the resultant signal integrated into circuitry that automatically adjusted the voltage to the wire to maintain a steady temperature. This means for maintaining constant temperature was substituted for prior art that measured electrical resistance of the wire to determine and maintain a constant temperature. In earlier techniques the wires were subject to oxidation and erosion and chemical attack by moving gaseous streams; consequently their diameters and hence their resistances changed for reasons other than their temperature. Thus, under the circumstances, electrical resistance was considered unsatisfactory as a measurement of temperature.
The overall methodology disclosed in U.S. Pat. No. 4,162,404 and earlier inventions were subject to limitations and exhibited disadvantages which will now be briefly discussed.
If sensor surfaces are heated by directly passing electrical currents therethrough, the selection of suitable sensor surfaces is limited to electrically conductive materials that, in addition to having the necessary attributes for particle detection, must also be satisfactory electrical conductors for their function. At the same time, the sensor surfaces must not oxidize in gaseous media to which they may be exposed at the high temperatures required for particle detection and they must also exhibit the necessary mechanical properties under all conditions to which they may be exposed. These restrictions have, as a practical matter, limited the choice of sensor material to platinum and a few of the other precious metals and alloys thereof. But, there are a number of other candidate materials for surface ionization sensors, including but not limited to the other precious metals and their oxides, tungsten oxide, and many of the transition metals, which have been excluded from use because they are unable to serve as their own resistive heating elements in an oxidizing or corrosive environment or provide adequate mechanical strength or both.
When an element of wire or ribbon of metal is being heated by passage of an electric current, and in the process becomes slightly more narrow at some place due to oxidation, abrasion or corrosion, then that "more-narrow" place subsequently becomes hotter than other portions of the wire or ribbon due to its greater electrical resistance. That elevated temperature then accelerates the oxidation or corrosion process at the location involved, resulting in the element's burnout although most of the element's material is still usable for particle detection.
Practical considerations of heating a sensor's catalytic surface with currents and voltages which are reasonable and available by conventional techniques mandate that the surface be a thin wire or ribbon or the like. As a result, the surface may be mechanically more fragile than desirable, particularly when operated at an elevated temperature in the presence of vibration and in turbulent airstreams. If there is mechanical vibration of the sensor surface vis-a-vis the collector electrode, the device functions as a capacitance microphone with an attendant electronic noise that limits the device's ultimate sensitivity. Moreover even if the wire or ribbon is quite thin and fragile, it still requires relatively high currents at relatively low voltages. Available power sources which satisfy these requirements tend to be relatively inefficient and the wiring and electronic connectors required to handle high currents are bulky and expensive.
U.S. Pat. No. 4,162,404 relates to means for controlling a catalytic sensor surface's temperature when the moving airstream varies in speed and temperature, and as the surface erodes, oxidizes or evaporates. The patent teaches use of a photocell and related circuitry to monitor the visible or infrared light emitted by the heated surface and to control the circuitry so as to maintain a constant intensity of light and hence a constant surface temperature. This, however, increases the sensor's size and complexity, and generally precludes its use in applications in which simplicity and cost are of paramount importance; furthermore, the technique cannot be used when high background levels of visible or infrared light are present.
It will thus be understood that a need exists for an instrument in which the catalytic surface which provides the pyrolytic and ionization processes can be selected and engineered without undue constraints imposed by size, form, electrical characteristics, or any requirements which lead to undue complexity, considering in particular the desirability of using off-the-shelf electrical electronic components in contrast to custom built components. For most industrial applications, the catalytic surface should be generally substantially more rigid than is normally possible to achieve with a wire or ribbon so that microphonic noises do not limit ultimate sensitivity. It is generally desirable that the configuration of the sensor be as simple as possible. Finally, it is desirable that the power required to heat the sensor to effective temperatures be at higher voltages and less current than prior art devices.